1. Field of the Invention
The present invention relates to methods and apparatus synchronizing exchanges of a telecommunications network, and in particular, a telecommunications network having communications carried out in pulse code.
2. The Prior Art
In a telecommunications network employing pulse code transmission of data, synchronization is necessary in order to insure satisfactory operation of the exchanges. it is necessary to have the bit rate incoming to an exchange agree with the bit rate which is determined by the local pulse generator within the exchange. it is necessary to eliminate slight phase fluctuations or jitters which arise on the transmission links, and in PCM telecommunications networks, it is necessary to carry out a so-called pulse frame compensation in order that all the time channels bearing the same ordinal number within the relevant pulse frame in the incoming and outgoing directions may coincide with one another in time.
A master-slave process of synchronization requires a central pulse generator with a high degree of accuracy. When the communications network is intact, no information losses occur with this type of synchronization. In the event of breakdown of the central pulse generator, in order to avoid or minimize information losses, it is necessary to provide substitution facilities for the central pulse generator or else the synchronized pulse generators within the exchanges must be sufficiently accurate to insure that they will not vary far from the desired pulse repetition rate until after a sufficiently long operating duration.
In addition to master-slave processes, two other processes are known in the prior art which allow prompt processing of switched information. In one case, the entire network can be operated asynchronously, in which case the individual pulse generators operate independently of each other, yet each possesses a relatively high frequency accuracy, so that the frequency of the individual pulse generators does not differ greatly. A buffer store is provided at the input of each of the exchanges to compensate for the phase difference between the pulse train of the incoming transmission line and the locally produced pulse train. This method leads to the disadvantage that, because of limited storage capacity of the buffer, it is sometimes necessary to suppress the entire storage contents or to read out the contents of the buffer a second time, with the result that a slip loss is introduced. Such slip losses also exist in master-slave systems where the central pulse generator becomes inoperative and no substitute pulse generator is provided.
Another system for maintaining network synchronization consists of the so-called phase averaging process. When this method is used, the pulse generators of the individual exchanges each have equal priority and are mutually synchronized together. Then the standard network frequency corresponds approximately to the mean value of the individual frequencies of all of the local pulse generators. No slip losses occur within such a network, although disturbances can impair the stability of the overall network. This makes it necessary to divide up large networks into smaller portions, so that individual portions then operate asynchronously with respect to each other, giving rise to the slip losses which are typical of asynchronous systems. In order to minimize slip losses in the phase averaging process, the pulse generators must possess a high degree of accuracy. The accuracy requirement lies between the accuracy required for the asynchronous network (approximately 10.sup.-10) and that required for the master-slave process (10.sup.-9). However, such accuracies are available only by means of relatively expensive pulse generators having a frequency standard based on atomic phenomena.